zoology exam 1

darwanian evolutionary theory

• Before Darwin’s theory, the first scientific explanation for evolution was from Jean Baptiste de Lamarck (1809).

  • Known as Lamarckism, this theory described evolution as the inheritance of acquired characteristics.

evidence for darwinian evolutionary theory

• Darwin supported his theory with key pieces of evidence. These include:

  • Perpetual Change

    • The world is an ever-changing place with hereditary continuity from past to present life.

    • This perpetual change can be easily seen in the fossil record, which reveals dramatic changes in the Earth’s environment.

Evolutionary trends

• Directional changes in features or patterns of diversity

• Fossil trends demonstrates Darwin’s principle of perpetual change, showing species arising and going extinct repeatedly

• more evidence for darwinian evolutionary theory

Common descent

• Life’s history is depicted as a branching tree: phylogeny.

• This means that all plants and animals have descended from an ancestral form. 

• Last universal common ancestor (LUCA) of living forms: over 4 billion years ago

  • Unicellular form resembling living bacteria

• more evidence for darwinian evolutionary theory

Homology and Phylogenetic Reconstruction

• Evidence for common descent.

• Characteristics inherited with some modifications from a corresponding feature in a common ancestor.

speciation

• how an ancestral species branch, or split, to form two or more descendant species

• Due to lack of gene flow between populations (reproductive barriers).

Allopatric Speciation

• Geographically isolated populations: can not interbreed but would.

• Separated populations evolve independently and adapt to their environment:

  • Reproductive barriers

• Allopatric speciation can occur via two types of geographical isolation:

  • Dispersal & colonization.

  • Vicariance

Dispersal and Colonization

• Process where individuals disperse from their previous population to colonize a new habitat.

  • This leads to genetic isolation that results in divergence between the two populations. 

Vicariance

• Process where some chance event physically separates a population into subgroups (physical barrier or mountain range/river)

Sympatric speciation

• Speciation that occurs among populations within the same geographical area.

• Individuals within a species become specialized for occupying different components of the environment.

Forces of Evolutionary Change

• The driving force behind evolutionary change is a change in the frequency of an allele in the gene pool of a population.

• This change in allele frequency can occur by any combination of these four processes:

  1. Mutation: continually introducing new alleles.

  2. Genetic Drift: random change of allele frequencies.

  3. Gene flow (migration): individuals leave one population and join another, bringing their alleles with them.

  4. Natural Selection: increases frequency of alleles that contribute to reproductive success

Mutation

• Creates new alleles, not only new combinations of alleles.

Genetic Drift

• Change in allele frequencies in population due to chance.

  • Causes allele frequencies to drift up and down randomly over time.

• Drift occurs in every population, in every generation.

  • Especially prevalent in small populations.

Gene flow

• Movement of alleles between populations.

  • Occurs when individuals leave one population, join another, and breed.

• Gene flow equalizes allele frequencies between source and recipient populations.

Natural Selection

• Can change both allelic frequencies and genotypic frequencies in a population. 

• Heritable variation leads to differential survival and reproduction:

  • Increases frequency of alleles that contribute to reproductive success in a particular environment.

Reproduction

• Two types:

  • Asexual reproduction

    • Creation of new genetically identical individuals.

  • Sexual Reproduction

    • Promotes diversity, enhancing long-term survival of the lineage in a world of perpetual change.

Sexual Reproduction

• Production of individuals from the fusion of gametes.

  • Bisexual (or biparental) reproduction: most common form – involves two separate individuals.

• Union of gametes from genetically different parents.

  • Offspring will have a new genotype.

Asexual Reproduction

• Production of new individuals without fusion of gametes.

  • Offspring all have the same genotype – clones.

• Can occur via a few different mechanisms, including:

  • Binary fission: division by mitosis used by bacteria and unicellular organisms.

  • Budding: new individual arises as an outgrowth (bud) from its parent.

Gametogenesis

• The production of mature gametes. 

• Spermatogenesis and oogenesis.

Spermatogenesis

• Occurs in testes.

• Takes place in differentiated germ cells within the seminiferous tubules (ST).

• Germ cells develop and produce male gametes with Sertoli (sustentacular) cells

• Outermost germ cells: spermatogonia (2n, increase in number by mitosis)

  • Primary spermatocytes (PS)

• PS move closer to the ST lumen – first meiotic division, secondary spermatocytes (SS)

• Each SS enters second meiotic division, produces two spermatids

• Spermatid transforms into mature spermatozoa

  • Reduction of cytoplasm; condensation of nucleus, formation of mitochondria, flagellar tail.

Oogenesis

• Oogonia (germ cells in ovary) increase in number by mitosis (2n)

• Grow in size: primary oocytes

• First meiotic division (13 weeks of fetal development):

  • Arrests in prophase I until puberty

  • First maturation (reduction) division occurs 

  • Secondary oocyte: large with most of the cytoplasm

  • Other cell: first polar body (very small)

• Second meiotic division:

  • Oocyte divides into a large ovum

  • Another in small polar body

• Each primary oocyte gives rise to only one functional gamete

• Meiosis II is completed only when the secondary oocyte is penetrated by a spermatozoon

Reproductive patterns

• Oviparous

  • Egg-birth. Lay eggs outside their body for development. Internal or external fertilization.

• Ovoviviparous

  • Egg-live-birth. Fertilized eggs remain in the oviduct or uterus while they develop. Embryos derive their nourishment from yolk stored within the egg.

• Viviparous

Live-birth. Fertilized eggs develop in the oviduct or uterus with embryos deriving their nourishment directly from the mother.

Developmental Studies

• Preformation vs. Epigenesis

  • Preformation

    • Young animals were pre-formed in eggs or sperm, would develop what was already there. (17^th and 18^th century naturalist philosophers)

  • Epigenesis

    • “ Origin upon or after”. A fertilized egg contains building material only.

Developmental Studies (cont)

• Development:

  • Describes the progressive changes in an individual from its beginning to maturity.

• Hierarchy of developmental stages: 

  • Cell types arise from conditions created in the preceding stages.

Early Development

• Cleavage: 

  • Embryo divides repeatedly. 

  • One large cell becomes many smaller cells: blastomeres.

• Blastula: 

  • End of cleavage. Zygote divided into hundreds or thousands of cells.

  • One layer of cells around a central fluid-filled cavity

• Gastrulation: 

  • Converts the spherical blastula into a two- or three-layered embryo (germ layers).

  • Body parts develop from one or more germ layers.

Formation of mesoderm – a third germ layer

• Diploblastic 

  •  Two germ layers (ectoderm and endoderm).

• Triploblastic 

  •  Three germ layers

• Mesoderm: between ectoderm and endoderm. 

  • Initial cells of mesoderm come from endoderm.

Further development

• Development of Coelom: 

  • A body cavity surrounded by mesoderm.

• Blastopore

  • First embryonic opening

• Origin of what will become the adult mouth:

  • Protostomes

    •  Blastopore becomes the mouth.

  • Deuterostomes

    • Blastopore becomes the anus; second embryonic opening becomes the mouth.

Amniotes and the amniotic egg

• Amniotes (reptiles, birds, and mammals)

  • Embryos develop within a membranous sac: the amnion.

• Amnion

  • A fluid-filled sac that encloses the embryo and provides an aqueous environment in which the embryo floats, protected from mechanical shock and adhesions.

Hierarchical organization of animal complexity

Protoplasmic, Cellular, Cell-tissue, Tissue-organ, Organ-system

Protoplasmic

unicellular organisms. Within a cell, protoplasm is differentiated into organelles capable of performing specialized functions.

Cellular

aggregation of cells that are functionally differentiated. A division of labor is evident.

Cell-tissue

aggregation of similar cells into definite patterns or layers and organized to perform a common function, to form tissues.

Tissue-organ

aggregation of tissues that form organs in a further step in complexity. Organs are usually composed of more than one kind of tissue and have a more specialized function than tissues.

Organ-system

organs working together to perform some function, producing the highest level of organization. Systems are associated with basic functions such as circulation, respiration, and digestion. 

Animal Body Plans

• Differ in grade of organization; body symmetry; n of embryonic germ layers; number of body cavities.

Animal Symmetry

• Refers to balanced proportions, or correspondence in size and shape of parts on opposite sides of a median plane.

• Spherical, Radial, Bilateral

Spherical symmetry

• Occurs in unicellular forms, rare in animals. Best suited for floating and rolling

Radial symmetry

• Can be divided into similar halves by more than two planes (oral and aboral surfaces). Occurs usually in sessile, freely floating, or weakly swimming animals. Interact with their environment in all directions

Bilateral symmetry

Animals that can be divided along a sagittal plane into two mirrored portions. Better fit for unidirectional movement

Tissue (Cellular components)

A group of similar cells specialized for performance of a common function

Histology

• study of tissues

4 kinds of tissues

• Epithelial, connective, muscular, and nervous.

• During embryonic development, germ layers become differentiated into four kinds of tissues

Complexity and Body Size

• Complex grade of animal organization permit and promote evolution of large body size.

• As animals become larger, body volume increases more rapidly than body surface area.

Small animals

• Absence of complex specialized tissue - rely on diffusion

• Body geometry that maximizes surface area and keep diffusion distances short (folding, invagination, flattening the body)

Larger, more complex animals

• Bring the environment functionally closer to each cell

• Use of internal transport and exchange systems with large surface areas